A compressed air energy storage system that uses a high pressure, isothermal air compressor/expander (C/E) has no carbon emission and is more efficient than a conventional system that uses fossil fuels. To be successful, the compressor/expander must be efficient and has high power density. However, there is a trade-off between efficiency and power density due to heat transfer. The authors' previous work has shown that by optimising the compression/expansion trajectories in a liquid piston C/E, the power density can be improved by many times without sacrificing efficiency. Yet, to achieve the optimised trajectory, this requires a large liquid piston pump/motor that often operates at low displacement, low efficiency regime. This study proposes that by combining the liquid piston with a solid piston actuated via a hydraulic intensifier, the pump/motor size can be reduced significantly. A case study shows that with an optimal intensifier ratio, the pump/motor size is reduced by 85%, the ratio between maximum and minimum displacements is reduced by 7 times, and the mean efficiency is increased by 2.4 times. A full cycle dynamic simulation shows that the intensifier decreases, for the same pump/motor size, the total cycle time for over 50%, thus doubling the power density of the compressor/expander.
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